1. Field of the Invention
The present invention relates to an LED drive circuit for driving an LED (light emitting diode), as well as to an LED illumination fixture, an LED illumination device, and an LED illumination system having an LED as a light source.
2. Description of Related Art
LEDs have such characteristics as low current consumption and long service life, and LED applications are expanding not only to display devices but to illumination fixtures and the like. In LED illumination fixtures, a plurality of LED units is often used in order to obtain the desired lighting intensity.
Common illumination fixtures usually use a commercial AC 100 V power supply, and in cases in which an LED illumination fixture is used in place of an incandescent bulb or other common illumination fixture, it is preferred that the LED illumination fixture also be configured to use a commercial AC 100 V power supply, as with a common illumination fixture.
In the case of applying dimmer control to an incandescent bulb, a phase control dimmer (commonly referred to as an incandescent light control) is used in which dimmer control can easily be applied to the supply of power to the incandescent bulb by a single volume element, by switching on a switching element (usually a thyristor element or triac element) at a certain phase angle of an alternating-current power supply voltage. However, in cases in which an incandescent bulb is dimmed by a phase control dimmer, flickering or blinking is known to occur, and normal dimming is not possible when a low-wattage incandescent bulb is connected to the dimmer.
The same type of phase control dimmer used for dimmer control of an incandescent bulb is preferably used in cases in which dimmer control is applied to an LED illumination fixture that uses an alternating-current power supply. FIG. 17 shows an example of a conventional LED illumination system whereby dimmer control can be applied to an LED illumination fixture that uses an alternating-current power supply.
The LED illumination system shown in FIG. 17 is provided with a phase control dimmer 2, an LED drive circuit having a diode bridge DB1 and a current limiting circuit 5, and an LED module 3. The phase control dimmer 2 is connected in series between an alternating-current power supply 1 and the LED drive circuit. In the phase control dimmer 2, when the knob (not shown in the drawing) of a semi-fixed resistor Rvar1 is set to a certain position, a triac Tri1 is switched on at a power supply phase angle corresponding to the set position. A noise prevention circuit using a capacitor C1 and an inductor L1 is also provided in the phase control dimmer 2, and terminal noise returning to the power supply line from the phase control dimmer 2 is reduced by the noise prevention circuit.
FIG. 19A shows an example of the voltage and current waveform of each component when an incandescent bulb 8 is operated by the phase control dimmer 2 (see FIG. 18), and FIG. 19B shows an enlarged view of segment P in FIG. 19A. In FIGS. 19A and 19B, VOUT2, I2, and I9 indicate the output voltage waveform of the phase control dimmer 2, the waveform of the current flowing to the triac Tri1 in the phase control dimmer 2, and the waveform of the current flowing to the incandescent bulb 8, respectively. In the example shown in FIGS. 19A and 19B, immediately after a trigger occurs and the triac Tri1 switches on, the current flowing to the triac Tri1 flows to the incandescent bulb 8 after oscillating several times between positive and negative.
FIG. 20A shows an example of the voltage and current waveform of each component when the LED illumination system shown in FIG. 17 is operated, and FIG. 20B shows an enlarged view of segment P in FIG. 20A. In FIGS. 20A and 20B, VIN2, I2, and I3 indicate the input voltage waveform of the phase control dimmer 2, the waveform of the current flowing to the triac Tri1 in the phase control dimmer 2, and the waveform of the current flowing to the LED module 3, respectively. In the example shown in FIGS. 20A and 20B, immediately after a trigger occurs and the triac Tri1 switches on, the current flowing to the triac Tri1 oscillates several times, and when the triac Tri1 switches on at a certain phase, the waveform becomes as if an oscillation had occurred, and normal dimming is not accomplished. As shown in FIG. 20B, which is an enlarged view of segment P in FIG. 20A, a process repeats in which the triac Tri1 switches off after the current flowing to the triac Tri1 has oscillated several times between positive and negative, the trigger then occurs again, the current flowing to the triac Tri1 oscillates several times between positive and negative, and the triac Tri1 then switches off. This is because the current flowing to the triac Tri1 decreases below a holding current when changing from positive to negative, and once the triac Tri1 has switched off, a period of time ensues in which the triac Tri1 does not respond for a certain time, and until the next trigger occurs after this period has elapsed, the current flowing to the triac Tri1 is below the holding current.
The LED illumination system shown in FIG. 21 is disclosed in Japanese Laid-open Patent Application No. 2006-319172. The LED illumination system shown in FIG. 21 is provided with a phase control dimmer 2, a diode bridge DB1, current holding means, rectifying/smoothing means, and an LED module 3. The phase control dimmer 2 is connected in series between an alternating-current power supply 1 and the diode bridge DB1, and the current holding means and the rectifying/smoothing means are provided between the diode bridge DB1 and the LED module 3.
The current holding means is composed of resistors R181 through R186, Zener diodes ZD1 and ZD2, transistors Q181 and Q182, and a capacitor C181. In the current holding means, in a case in which the power supply voltage outputted from the alternating-current power supply 1 is 100 V or lower, the transistor Q182 switches on and a current is applied which corresponds to the holding current of the triac Tri1 in the phase control dimmer 2. The transistor Q182 switches off in a case in which the power supply voltage is not 100 V or lower. The transistor Q182 applies a current (about 30 mA) such that the current flowing through the triac Tri1 in the phase control dimmer 2 does not drop below the holding current.
However, in the current holding means described above, the time during which the collector current of the transistor Q182 is flowing is the period from the time the transistor Q182 switches on until the time the transistor Q181 switches on, and the transistor Q181 switches on when the Zener diode ZD1 switches on after the triac Tri1 in the phase control dimmer 2 has switched on. In other words, in such cases as when the triac Tri1 in the phase control dimmer 2 rises steeply, or when the power supply voltage of the alternating-current power supply 1 is high, the transistor Q182 is on for only a short period, and a current that does not drop below the holding current of the triac Tri1 is flowing for only a short time. Cases therefore occur in which the triac Tri1 cannot switch on.